1. Field of the Invention
The present invention relates to a device that includes more than one photosensitive body having a surface which is scanned with laser beams to form an electrostatic latent image and that forms a color image on a sheet-like medium by sequentially overlapping and transferring the manifest image (toner image) of each color developed on each photosensitive body, on the sheet-like medium.
2. Description of the Related Art
Generally, in color image forming devices such as electro-photographic printers, a color image is formed on a transfer paper sheet (sheet-like medium) by overlapping images of plural colors (for example, four colors including yellow (Y), magenta (M), cyan (C) and black (BL)). For that reason, the color image forming device includes more than one printing unit to print an image of each color. Images of respective colors are sequentially transferred on a transfer paper sheet conveyed to each of the printing units.
In each of the printing units, a photosensitive body, the entire surface of which is uniformly and electrically charged, is exposed to a laser beam from the optical system to form an electrostatic latent image on the surface. The electrostatic latent image is developed using a developer (toner) of a color corresponding to each printing unit. Then the toner image developed on the photosensitive body is transferred onto the transfer paper sheet conveyed. A color image is formed on a transfer paper sheet by sequentially overlapping a toner image of each color on a sheet of transfer paper in each of the printing units. Finally, the toner image is fixed on the transfer paper sheet.
In such a color image forming device, since an image is completed by overlapping sequentially images of photosensitive bodies on a sheet of transfer paper, it is needed to locate respective color images at the same position. Then generally, various techniques for adjusting an image recording start position or image recording width have been developed as a method of positioning in a main scanning direction to transfer respective color images at the same position on a transfer paper sheet.
For example, in order to adjust an image recording width (or to correct a magnification error), each printing unit forms toner images in at least two positions in the main scanning direction of a transfer belt which conveys transfer sheets. Then the position of each toner image is detected by means of a CCD camera or the equivalents. The magnification error is compensated by adjusting the frequency of image clock pulses for creating an image signal, based on the positional deviation amount between a reference position and the position of a toner image actually formed on the transfer belt, so that image recording widths in the printing units coincide.
In the device that performs a photosensitive exposing process using the above-mentioned laser beam, the f.theta. characteristics in the optical systems differ with the positional accuracy of each of the lenses (f.theta. lenses) or mirrors in the optical systems. Therefore, even if the irradiation positions where the laser beams from the optical systems start drawing an image coincide and the irradiation positions where the laser beams from the optical systems finish drawing the image coincide, the laser beams from the optical systems may occupy at different positions in the intermediate region between the starting position and the finishing position, thus causing color slurring in the same region.
In a prior art, as disclosed in the Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 2-282763, for example, a technique is proposed by which color slurring (uniform velocity error) at the portion between the start portion and the end portion of plural images overlapped, which is caused by the differences of f.theta. characteristics due to the positional accuracies or the like of the optical systems, is corrected by varying the irradiation timing of a laser beam according to the position in the main scanning direction of a deflected laser beam.
The optical system which irradiates laser beams onto a photosensitive body includes a laser light source which is driven according to image signals to emanate laser beams and a rotary polygon mirror (hereinafter, often referred to as a polygon mirror) which deflects the laser beam from the laser light source to scan the surface of a photosensitive body in the main scanning direction using the laser beam.
In the above-mentioned prior-art technique, the uniform velocity errors due to the positional accuracy of each of the lenses or mirrors in the optical systems are corrected, but the characteristic of each of mirror surfaces (mirrors) of the polygon mirror is not considered. It is basically impossible to maintain the surface accuracy of each mirror surface of the polygon mirror in each printing unit at the same level. Improving the surface accuracy leads to an increase in cost of the polygon mirror. However, the error of the surface accuracy cannot be ignored to improve the print quality of color images. Therefore, because of an increasing demand for higher resolution, color-oriented trend and higher quality in recent years, it has been desired to correct certainly uniform velocity error accompanied with the characteristics of each optical system by performing a fine laser beam irradiation position control, in consideration of the surface accuracy error of each mirror surface of the polygon mirror in addition to the positional accuracies of lenses and mirrors in the optical systems.
Originally, in order to correct magnification errors, it is possible to obtain error data taking into account the uniform velocity error by detecting the position of a toner image formed on the transfer belt, as described above. However, it is actually difficult to obtain an error associated with the surface accuracy error of each mirror surface of the polygon mirror based on the toner image on the transfer belt because it must be recognized that with what mirror surface a toner image is drawn.